Network configuration for fastest possible speed

I have two "experts" telling me conflicting facts. The first expert says the locking down a NIC and switch port to 100 full duplex will make the network go as fast as possible. The other expert says that he has seen better network speed with leaving both ends set to auto negotiate.

Personally to me it seems that 100 FD would give the fastest performance because the devices would not have to “talk” about the speed and duplex rate, but I didn’t want to go through the lengthy process to prove the facts. If anyone has any first hand experience with this question, please include the type of devices you have seen this scenario tested on.

It doesn't really matter which you do, because the two ends of the cable only "talk" about how fast they're going to communicate when they first connect - when you plug a cable into a switch/hub, you'll often notice a short delay before the light comes on, or on some older equipment, you'll get just the basic connect light, then the speed/duplex indicator will indicate how fast the link is.

Leaving devices on auto-negotiate is usually the best option, because then if you change network equipment (say your primary breaks down, and you only have an old 10Mb hub as backup) then you'd have to re-visit all those machines to reset the speed/duplex.

Manual is the best way to guarantee maximum throughput. I have seen autonegotiation devices drop to Half duplex under heavy load because the device was too busy to send the control packets.
Also keep in mind that Autonegotiation is designed to work only if BOTH devices are set to auto. Otherwise, you will likely end up with 100 Mbps Half duplex on most of today's devices.

Virtually every switch vendor out there recommends locking down speed and duplex except in 2 instances:
- Gigabit. The autonegotiation also includes flow control so is needed.
- "Guest" ports. If you don't know that the same device is always coming back to the same port, you *may* want to set it to autonegotiate.

Some people have complained that this means the end users now have to manually configure their PC's to manual 100/FD, but if you maintain that as a standard, you never need to worry, even if they move.

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To auto-negotiate or not is really a question of choice I think - i've worked in several disparate environments, where when a user moved he might be on 100Mb, he might not be, so hard-coding produced extra work. If you're in a heterogenous environment, then hard-coding would be easy, and wouldn't result in any extra work for you.

I guess my understanding of the speed/duplex negotiaion might also be wrong - but maybe not - I thought that once both ends were in link active state, they didn't bother with any further speed/duplex negotiation.... with the exception of flow control information.

I personally always lock servers to speed/duplex settings, but thats because they're directly under my control, and I know what they're being plugged into, and I'm always in the server room anyway.

Yeah my understanding of auto-negotiation is that one both cards are in "link-enabled" state (ie the link light comes on) they don't negotiate speed/duplex again until you unplug the cable and plug it back in. So hard-coding wouldn't make a difference to overall network speed.

I was going to split the points on this question, but I didn't see how to do that. I thought you should get half the points because of your participation. If there is a way to send those points to you, let me know how.

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